Synthesis of Aramid Polymers (Polymerization Process)

The world capacity of para-aramid production is estimated at about 41,000 tons/year in 2002 and increases each year by 5–10 %. In 2007 this means a total production capacity of around 55,000 tons/year.

POLYMER SYNTHESIS

Aromatic polyamides are synthesized most frequently from an aromatic di-amine and an aromatic di-acid or di-acid chloride by the following reaction:

Low Temperature Poly-Condensation

A classic example of low-temperature polymerization is the preparation of poly (p-phenylene terephthalamide) (PPD-T) from p-phenylenediamine (PPD) and terephthaloyl chloride (TCl) in an amide solvent as shown below:

This reaction can also be carried out by several polymerization methods like interfacial polymerization and solution polymerization. The most convenient method is low-temperature polycondensation in a solvent. In general, the initial monomer concentration is in the range of 5 ± 20 wt. %, depending on the polymer solubility and viscosity. Anhydrous diamine and diacid chloride are used in equimolar quantities to ensure maximum polymer molecular weight. One of the monomers, usually the diamine, is dissolved in an amide solvent. The second monomer is then added to the monomer solution to initiate the polycondensation. The reaction is then allowed to proceed in a tinder dry nitrogen atmosphere at -10 to 60 oC for a period of several minutes to several hours. As polycondensation proceeds, the reaction mixture will become increasingly viscous and the polymer may precipitate at a certain point. After allowing the reaction to continue for some time, the polymer is finally isolated from the reaction mixture by a nonsolvent such as water. It is thoroughly washed, neutralized, and then dried. In some cases, the polymer may remain soluble in the polymerization solvent throughout the course of reaction. Such a reaction mixture can be processed directly to form fibers or other shaped products.
Two major types of solvents for the low-temperature polycondensation of aromatic polyamides: halogenated nonaromatic hydrocarbons and organic amide solvents. Examples of the nonaromatic hydrocarbons are chloroform, methylene chloride, methyl ethyl ketone, acetonitrile, tetra-methylene sulfone, dimethylcyanamide, and propionitrile. The amide solvents which are most often used in the synthesis of aromatic polyamides include dimethyl acetamide (DMAc), N-methyl-2-pyrrolidone (NMP), hexamethyl phosphoramide (HMPA), and tetramethyl urea (TMU). In many instances, a small amount of alkaline and alkali earth metal salts such as lithium chloride, lithium hydroxide, calcium chloride, calcium hydroxide, is used in combination with the solvent to increase the polymer solubility or to neutralize the acid chlorides produced in the reaction.

Other Synthesis Methods

Several alternate methods have been reported for the polymerization of aromatic polyamides. Preston and Hofferbert (1979) attempted to synthesize aromatic polyamides by a phosphorylation reaction in DMAc/5% LiCl in the presence of triphenylphosphite and pyridine. This method gave only modest polymer molecular weight in the synthesis of PPD-T. Higashi and Taguchi in 1981 refined the phosphorylation reaction by reacting an aromatic diamine and an aromatic dicarboxylic acid in NMP/LiCl in the presence of triphenylphosphite and poly (4-vinylpyridine). The refinement also included the addition of various pyridine derivatives. They obtained PPD-T with a high inherent viscosity of 4.5 dL g-1 by the polycondensation of p-phenylenediamine and terephthalic acid in NMP/LiCl/CaCl2/pyridine/triphenylphosphite at 1.2% polymer concentration.